Photosynthesis

Key to living in the extreme desert soils of eastern Antarctica: a chemolithotrophic lifestyle

Abstract:

Mitchell Peninsula is located at the south of the Windmill Islands, Eastern Antarctica. It is described as a nutrient poor, extreme polar desert and limited knowledge on the microbial diversity of the soils in this area exists. We examined the microbial taxonomic composition and metabolic potential of Mitchell Peninsula soils using 16S metagenomics and shotgun metagenomics. We found the site to be a potential biodiversity hotspot, containing a high abundance of Candidate Phyla WPS2 and AD3. Subsequently, differential binning was used to recover 23 draft genomes, including 3 genomes from WPS-2 and two from AD3. Further analysis of the metagenome revealed a novel Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) gene to be abundant in the bacterial community, despite a lack of evidence for photosynthesis related genes. We believe that unlike many other Antarctic regions, chemolithautrophic carbon fixation via CBB cycle is the dominant carbon fixation pathway, hence this pathway is providing the key to survival is this very dry, hostile environment.

The smallest, most abundant phototroph in the world, Prochlorococcus, dominates the base of the food web in the “Outback” of the world’s oceans, the nutrient-depleted ocean gyres. This unicellular, marine cyanobacterium, unknown only 30 years ago, is an oligotrophic specialist with a streamlined genome and reduced cellular requirement for the limited resources available in this environment. Based on physiological and molecular analyses of isolated strains from different oceans and depths, two broad groupings of Prochlorococcus were characterized: high- and low-light adapted “ecotypes”. Within these broad groupings are many subclades, some of which have been shown to dominate under certain temperature and light conditions. Through additional culture-based studies, my lab has been exploring nutrient physiology and other physiological characteristics that may contribute to the ecology and evolution of other Prochlorococcus subgroups. Some subgroups have the capacity to utilize nitrate, which was not the case for the initial isolates of Prochlorococcus, and others differ in their pigmentation. We have also found that Prochlorococcus regulates its uptake velocity and specific affinity for inorganic and organic phosphorus under P stress conditions. Examining the physiology, ecology and genomics of Prochlorococcus isolates and natural populations is providing insights into how these tiny photosynthesizing cells create a stable, yet invisible forest in the deserts of the world’s oceans.

Effect of Low Temperature on Tropical and Temperate Isolates of Marine Synechococcus.

Abstract:

An abundant and globally occurring marine picocyanobacterium, the genus Synechococcus is an important player in oceanic primary production and global carbon cycling. In the complex marine environment, this widespread organism has evolved to successfully colonize and inhabit different environmental niches. Their biogeographic distribution suggests that Synechococcus ecotypes exhibit thermal niche preferences. Temperature is a key environmental variable and the elucidation of the temperature stress acclimation in members of this genus can shed light on the molecular mechanisms involved in their adaptive capability. The growth of four representative Synechococcus isolates of various ecotypes from tropical and temperate regions were monitored under various temperature conditions. This revealed drastic differences in growth rates in correlation with their thermal niche preferences. The temperate strains CC9311 and BL107 displayed higher growth rates at lower temperatures while tropical strains WH8102 and WH8109 grew better at higher temperatures. In order to further elucidate their thermal niche preference, the molecular factors influencing the temperature-related growth patterns were explored through global proteomic analysis of WH8102 and BL107. Whole cell lysates of the strains grown at different temperature conditions were fractionated using 1D SDS-PAGE and analysed using label-free quantitative proteomics. Protein identifications provided 27% and 40% coverage of the whole genome for WH8102 and BL107, respectively. Quantitation of protein expression revealed 22% and 20% of the identified proteins were differentially expressed in WH8102 and BL107, respectively. The results were further investigated using qRT-PCR and PAM fluorometry. Differential expression revealed that low temperature appeared to have a significant effect on the photosynthetic machinery. The light harvesting components, phycobilisomes exhibited a reduced expression which could be the result of protein degradation due to photo-oxidative damage and/or as a mechanism to restore the energy balance disturbed as a consequence of low temperature. The lowered phycobilisome expression is found to be a common low temperature-related response between the tropical and temperate isolates. Within the photosynthetic reaction centres, differences in the expression of some core proteins were observed between the two isolates. The expression of core proteins could correlate with the efficiency of repair mechanisms involved in the replacement of photo-damaged core proteins. This differential expression sheds light on the underlying factors which potentially influence the differences in the thermal ranges of tropical and temperate isolates.

Ocean warming is expected to affect marine microbial phototrophs directly by influencing their metabolism and capacity for photosynthesis as well as indirectly through altering the supply of resources needed for growth. In turn, changes in phototrophic community composition, biomass and size structure are expected to have cascading impacts on export production, food web dynamics and fisheries yields, as well as the biogeochemical cycling of carbon and other elements. As a result, temperature is a critical parameter in coupled climate-ocean models because it influences not only the magnitude, but also the direction of future ocean productivity.

This seminar presents data from several recent oceanographic voyages to suggest that the statistically significant relationships found between temperature and carbon fixation of contemporary ocean microbes is confounded by the availability of co-varying light and nutrient resources, and challenges the notion that satellite-derived sea surface temperature is a suitable proxy for tracking changes in upper ocean biogeochemical function. It will also present laboratory data which demonstrates that thermal selection of photosynthetic microbes (over >100 generations) results in phenotypic trait evolution and shifts in photosynthesis:respiration. Collectively, these data show non-linearity in metabolism of photosynthetic microbes in a warming ocean, pointing to increased variability of responses and potentially less predictability in models.

The ocean around Antarctica is not just cold, it’s also dark for a large part of the winter. This means that carbon fixation by photosynthesis is inhibited during the polar winter. We used metaproteomics to reconstruct the ecology of microbes at the surface of the Southern Ocean near the Antarctic Peninsula, for both winter and summer seawater samples. Metagenomics (community genomics) tells us what kinds of genes are present. Metaproteomics goes a step further and determines which proteins (including enzymes) are actively being produced by microbes within a community. Therefore, we can use this approach to reconstruct microbial processes used for carbon fixation, nutrient acquisition, and other metabolic pathways. We found that ammonia-oxidising archaea were dominant at the Southern Ocean in winter, with the detected proteins indicating that they had a major role in ‘dark’ (light-independent) carbon fixation at the surface. In summer, by contrast, these autotrophic archaea were undetectable at the ocean surface, when photosynthesis by algae was the major route of carbon fixation. SAR11 bacteria (Pelagibacter spp.) were prevalent in both winter and summer, and detected proteins indicate that ATP-dependent uptake was important for the acquisition of nutrients by these heterotrophs, including simple organic compounds such as amino acids and taurine. Flavobacteria (especially Polaribacter) were more prevalent in summer, and the detected proteins show that these heterotrophic bacteria use exoenzymes to target complex biomolecules (polypeptides, polysaccharides) released from decaying algae. Overall, metaproteomics of the Southern Ocean surface has allowed us to identify the similarities and differences between winter and summer microbial communities, as well as which particular nutrients are being targeted by individual groups of bacteria and archaea.

This remarkable compound, found in stromatolite-inhabiting cyanobacteria from Shark Bay, Western Australia, can absorb light further in the red region of the electromagnetic spectrum than any of the other known chlorophylls.
This work was a truly collaborative effort between Sydney-based (University of New South Wales, the University of Sydney and Macquarie University) and international researchers (University of Munich).